1. Field of the Invention
This invention relates to a system for controlling the rotation of a rotary mechanism such as a propeller housing in a Z-type propulsion apparatus for a watercraft.
2. Prior Art
In recent years, harbors have become much crowded with vessels of a large size. Therefore, it has been required that tug boats should operate to move the vessel toward and away from the shore in a safe and rapid manner. For this reason, there have now been extensively used tug boats equipped with a Z-type propulsion apparatus which can easily vary the direction of propulsion over the range of 360 degrees. FIG. 1 shows one conventional system for controlling the rotation of a propeller housing of such a Z-type propulsion apparatus mounted on a tug boat. There is provided in the control system a pivotal steering arm 1 for commanding the propeller housing to rotate by a desired amount of angle in a selected direction. The steering arm 1 is provided with an angular position detector 2 for detecting the angular position of the steering arm 1. There is also provided another angular position detector 3 for detecting the angular position of the propeller housing. Both outputs of the detectors 2 and 3 are supplied to a servo-control circuit 4 which in turn generates a servo-control signal to a servo-motor 5. An output shaft of the servo-motor 5 is connected to an input shaft of a hydraulic pump 6, and input and output ports of the pump 6 are connected to a pair of ports of a hydraulic cylinder 7 via the respective connecting tubes. The piston of the cylinder 7 is connected to an input shaft of another hydraulic motor 8 through a piston rod 7a and an arm 8a so that the input shaft of the hydraulic motor 8 rotates in accordance with the movement of the piston. The hydraulic pump 8 is activated by an electric motor 9 and supplies the pressurized oil to a hydraulic motor 10, the amount and direction of flow of the oil varying in accordance with the angular position of the input shaft of pump 8. A worm 11 mounted on an output shaft of the hydraulic motor 10 is engaged with a worm wheel 12 which is coaxially mounted on the propeller housing.
With this construction, the propeller housing rotates in synchronism with the angular movement of the steering arm 1 and starts to rotate and stops smoothly by virtue of the provision of the servo-motor 5, hydraulic motor 6 and hydraulic cylinder 7. This conventional control system is however rather complicated in construction and it is therefore difficult to reduce its size as well as its manufacturing costs.
FIG. 2 shows another conventional control system specifically designed to overcome the above-mentioned disadvantages. In this control system, an electronic servo-control circuit 13 having a delay function is provided to directly control a servo-valve 16. The servo-valve 16 controls the amount and direction of flow of oil from a hydraulic pump 15, which is driven by an electric motor 14, and supplies the controlled oil to a hydraulic motor 17. This hydraulic motor 17 is to drive the worm 11.
This control system is however liable to be adversely affected by the variation of ambient temperature and noises since the servo-control circuit 13 comprises electronic amplifiers. Another deficiency of this control system is that the propeller housing begins to rotate with much delay when the signal level of the output signal S of the servo-control circuit 13 is small. As is seen from FIG. 3, the output signal S at a small signal level S2 rises at a less inclination than that at a large signal level S1. The propeller housing is driven by the output signal S through a control mechanism having a dead zone B. The propeller housing therefore begins to rotate in response to the small level signal S2 with a delay period t2 which is greater than a delay period t1 corresponding to the large level signal S1. Further, the propeller housing is subjected to vibration during a deceleration of the rotation of the same if the time constant of the servo-control system is set to a small value in order to avoid hunting of the propeller housing. The vibration can be avoided by setting the time constant to a large value, in this case however the propeller housing may be subjected to the hunting.